This invention relates to a hydraulic system for a marine propulsion unit and more particularly to a system that insures against unwanted downward movement due to valve leakage.
As is well known many marine propulsion systems, particularly ones having larger displacements employ hydraulically operated trim and tilt controls. These systems generally permit trim adjustment when the watercraft is in motion and tilting up out of the water for trailering or service. In addition they generally incorporate a pop up damping arrangement that permits the propulsion unit to pop up when an underwater obstacle is encountered to prevent damage and return to the trim adjusted position when it is cleared.
One such arrangement is shown in Published Japanese Application, publication number Hei 07-69289, published Mar. 14, 1995. As shown in that publication, the tilt and trim arrangement comprises a clamp bracket fixed to the watercrafthull and on which a propulsion unit is pivotally supported for the trim and tilt operation. This is accomplished by a tilt cylinder mounted with its axis extending in a generally vertical direction and capable of expanding and retracting in the axial direction. The lower end of the cylinder is pivotally supported by the clamp bracket through a lower pivot and its upper end is pivotally connected to the propulsion unit through an upper pivot. A pressurized oil control system for controlling oil delivery to accomplish the desired motion.
The tilt cylinder includes a cylinder body forming a large cylinder bore into which a large trim piston is fitted. A small cylinder bore is formed around the axis in a part of the cylinder body above the large cylinder bore with its upper end externally opened and its lower end communicating with the large cylinder bore. A cylinder tube with its upper end closed is fitted into and inserted through the small cylinder bore and connected to the large piston. A small piston is fitted into a separate cylinder bore in the cylinder tube. A piston rod is provided, which has an end extending upward from the small piston through the closure, is pivotally connected to the propulsion unit by the upper pivot.
However, when the trim cylinder is operated to expand/retract in order to adjust the trim position of the propulsion unit, the propulsion unit swings up and down as the piston repeatedly slides with respect to the inner peripheral surface of the cylinder bore through the sealing body with friction, as described above. The same occurs during repeated tilt up and down operations as well as popping up and return operations as occur when underwater obstacles are encountered and cleared. Therefore, extended and repeated use of the unit may often produce a minute foreign matter resulting from the wearing of the inner peripheral surface of the cylinder bore or the sealing body.
The fluid control system includes a shuttle valve assembly that incorporates a pair of oppositely opening check valves that are positioned on opposite sides of a shuttle piston. When one side is pressurized its associated check will open to supply fluid pressure to effect operation of the unit to move the propulsion unit inn the desired direction. The shuttle piston will then shift to forcibly open the other check valve to provide a return fluid path to the pump. When the desired position is reached the pump is stopped and the check valves close to retain the propulsion unit in the desired position. In the extreme case this may be the tilted up out of the water position where it is intended to be maintained for a long time, for example when in storage.
However the aforenoted foreign matter may either cause pitting of the ball check valve, wear of its seat or become lodged between the desired sealing surfaces. This can result in minute leakage that will cause the propulsion unit to gradually move from its desired position, an obviously unsatisfactory condition. Although frequent servicing of the unit and inspection could avoid this problem, this is not always done and can be expensive.
It is therefore a principal object of this invention to provide a simple and low cost hydraulic solution to this problem that reduces the necessity of frequent and expensive servicing.